Processes for producing dispersion-modified alloys



United States Patent US. Cl. 75-205 8 Claims ABSTRACT OF THE DISCLOSURE Powder metallurgy methods heretofore known for producing chromium-iron group metal alloys dispersionstrengthened with a particulate refractory oxide such as thoria have included the steps of compacting a powder containing chromium and the iron-group metal, with the refractory oxide dispersed therein, to a green billet of low strength and high porosity, canning the billet in a non-contaminating container such as a mild steel can, and hot-working the canned billet, as by extrusion, to theoretical density. The densified alloy so-obtained has an objectionably high excess oxygen content. By first heating the starting powder with nitrogen at 650 to 890 C. to convert the chromium to chromium nitride, CrN, then compacting it to a green billet, canning the billet, denitriding it with hydrogen at 900 to 1000" C., and hotworking it to a dense metal product while still in the can, the undesirable excess oxygen pick-up is avoided and alloy products having a clean microstructure are obtained.

More particularly the invention is directed to nonporous, solid alloys comprising chromium and an irongroup metal, preferably nickel, the alloy having dispersed substantially uniformly therein about from 0.05 to 20% by volume of particles of a refractory metal oxide having a free energy of formation at 1000 C. greater than 103 kilocalories per gram atom of oxygen, preferably thoria, and having an average particle size of less than about 80 millimicrons, and being further characterized by having a clean microstructure at 200x magnification.

The invention is further particularly directed to powders for use in making the alloys, the powder comprising as essential components (a) an iron-group metal, preferably nickel, (b) chromium nitride of the formula CrN, and (c) about from 0.05 to 20% by volume, based on the composition without the nitrogen of the chromium nitride, of particles of a refractory metal oxide having a free energy of formation at 1000 C. greater than 103 kilocalories per gram atom of oxygen and an average particle size of less than 80 millimicrons, the oxide preferably being thoria, said essential components being codispersed substantially uniformly in the powder particles, and said powder particles being substantially free of metal carbides and of metal oxides other than the refractory oxide.

The invention is further particularly directed to the steps, in a process for producing the novel powders comprising (1) providing a powder comprising as essential components (a) an iron-group metal, preferably nickel, (b) chromium, and (0) about from 0.05 to 20% by volume of particles of a refractory metal oxide having a free energy of formation at 1000 C. of greater than 103 kilocalories per gram atom of oxygen and having an average particle size of less than 80 millimicrons, the oxide preferably being thoria, said powder particles being substantially free of metal carbides and of metal oxides other than the refractory oxide, and (2) heating said powder in nitrogen at a temperature in the range of 650 to 890 C. until at least 98% by weight of the chromium present has combined with nitrogen to form chromium nitride of the formula CrN.

The invention is still further directed to processes in which a powder prepared by steps (1) and (2) as just described is subjected to the further steps of (3) compacting at a temperature below about 50 C. to from 50 to of theoretical density, (4) enclosing the compact in a non-contaminating container before it has come into contact with an oxygen-containing gas, above C., (5) displacing the atmosphere in said container, at a temperature in the range of 900 to 1000 C., with a medium selected from the group consisting of vacuum, flowing inert gas, and flowing hydrogen, whereby nitrogen in the chromium nitride is removed, and (6) continuing said displacement until all said nitrogen has been lI'- moved.

The invention is also directed to processes as just described in which, following the nitrogen removal in step (6), the compact, while still enclosed in the non-contaminating container and in a non-reactive environment, is hot-worked to substantially theoretical density.

The preparation, by powder metallurgy methods, of alloys containing chromium and iron-group metals, which alloys are dispersion-strengthened with particulate refractory oxides such as thoria, has been described in U.S. Patent 2,972,529, issued Feb. 21, 1961, to Alexander, Iler and West. By reason of the presence of the dispersed oxides the alloys have remarkably improved high temperature strength properties. The patent describes a novel method wherein the hydrous oxides of an irongroup metal and of chromium are coprecipitated with the reinforcing oxide particles, the precipitate is dried, and the chromium and iron-group metal oxides are chemically reduced to the corresponding metals containing the nonreduced, refractory oxide particles in a dispersed condition. This chemical reduction is effected at elevated temperatures, as high as 1100 C., using hydrogen or various carbonaceous agents, for such prolonged times as eighty hours. The reduced powder so obtained has hitherto been consolidated into solid metal form by known powder metallurgy techniques.

In the Alexander et al. Patent 2,972,529, it is pointed out that the so-called excess oxygen content-that is, oxygen in excess of that combined in the refractory oxide, must be extremely low. Broadly, the excess oxygen must be below 2% by weight and preferably below 0.1%. The methods taught in the patent are quite capable of producing products having excess oxygen below these limits in the as reduced condition.

Unfortunately the above-described powders in the asreduced condition have a very strong afiinity for oxygen, particularly at elevated temperatures. For products having very high surface areas this afiinity may be so great as to make the products pyrophoric upon exposure to air,

- but even for powders of low surface area the oxygen pick-up is objectionably high.

In particular, it has now been found that the powder metallurgy methods heretofore available for consolidating the reduced powders to useful compacted solid metal products lead to excess oxygen contents which are above desirable limits. Briefly, these methods include compacting the powder to a green billet having low strength and high porosity, canning the billet in a non-contaminating container such as a mild steel can, and hot-working the billet, while still canned, as by extrusion, to a product having substantially theoretical density. Even when the starting powder in this sequence of operations has an excess oxygen content well below permissible limits, the final, densified alloy has excess oxygen which is objectionably high. Such oxygen is present as occlusions or agglomerates of metal oxides which tend to break up upon subsequent working, thus weakening the metal and giving anisotropic strength properties. To the extent that such occlusions are present the metal, after metallurgical polishing, will show a characteristically dirty microstructure when examined microscopically at a magnification of 200 times. It has not hitherto been possible to achieve a clean microstructure at this magnification, using consolidation methods heretofore available in the art.

Now according to the present invention novel, practicable processes are provided whereby dense metal products having clean microstructures can be produced. It has been found that in the dispersion-modified, chromiumcontaining powders above-described, the chromium can be converted to chromium nitride, CrN, by heating it with nitrogen at a temperature of 650 to 890 C. The powder can then be consolidated by the conventional techniques of compacting to a porous billet and canning this billet, after which the nitrogen can be quantitatively removed by heating the billet, while still in the can, in a flowing stream of hydrogen or inert gas or in vacuum, at a temperature in the range of 900 to 1000 C. The denitrided product can be hot-worked in the can, as by extrusion, to a dense metal product which is no longer susceptible to oxygen pick-up and which has the desired clean microstructure.

In practicing the invention care should be used in the selection of the starting powder. Such powders are not new, being described, for example, in the above-mentioned US. Patent 2,972,529, but precaution should be exercised to avoid undesirable occlusions, such as oxides of the metal components. These components are the chromium and the iron-group metal, the latter being iron, cobalt or nickel or mixture of two or all three of these. The chromium content should be in the range of to 40% by weight, preferably to There may also be present other metals, such as tungsten, molybdenum, manganese or vanadium, in minor amounts.

The particulate refractory oxide dispersed in the metal must be an oxide having a free energy of formation (sometimes designated as negative), at 1000 C., greater than 103 kilocalories per gram atom of oxygen. This group includes the oxides of aluminum, cerium, hafnium, uranium, magnesium, thorium, berylium, lanthanum, calcium, and yttrium. One or more of this group can be used. Thoria is preferred. The refractory oxide must be extremely finely divided, having an average particle size less than about 80 millimicrons and preferably less than millimicrons. The proportion of refractory oxide present should be about from .05 to 20 percent by volume preferably 0.5 to 5%.

The iron-group metal or metals, chromium, and refractory oxide or oxides must be codispersed in the powderthat is, there should be a substantially uniform dispersion of each in relation to the others. Such codispersion can be achieved, for example, by the coprecipitation methods described in US. Patents 2,972,529.

The starting powder particles must be substantially free of metal carbides and of metal oxides other than the refractory oxide. Freedom from undesired metal oxides, such as chromium oxide, can be assured by reducing such oxides in a flowing stream of very dry hydrogen at elevated temperatures up to about 1100 C. for extended periods of time, or if desired, by heating with a carbonaceous reducing agent such as methane. Freedom from carbides can be accomplished by reducing solely with hydrogen, or, if carbon is used, limiting the quantity of carbon to that necessary to deoxidize any undesired metal oxide present.

Having selected a suitable starting powder the next step is to heat this powder in nitrogen at a temperature in the range from 650 to 890 C., preferably 700 to 800 C., until at least 98% by weight of the chromium present has combined with the nitrogen to form chromium nitride of the formula CrN. This can advantageously be done immediately following any prior deoxidation step,

and in the same equipment. Control of the temperature within the range stated permits the formation of CrN to the exclusion of Cr N, the latter being more difiicult to denitride subsequently in the process and therefore undesired. The nitridation proceeds quite rapidly, but sufiicient time should be allowed to ensure substantially complete conversion.

After nitridation of the chromium is complete, the powder is cooled to a temperature below C., preferably room temperature, and then compacted to from 50 to 75%, preferably 60 to 70%, of its theoretical density, to form a green billet or compact. Compaction methods are well known in the powder metallurgy art and any of these, such as hydrostatic compaction in a flexible boot, can be used.

Next, the compact or green billet is enclosed in a noncontaminating container, before it has come into contact at elevated temperatures with any oxygen-containing gas, such as air. By elevated temperature is meant temperatures in excess of 100 C. The nitridation of the chromium protects the composition against excessive oxygen pick-up at ordinary ambient temperatures, but at elevated temperatures, there is danger of oxidation of the iron-group metal. The art is familiar with suitable non-contaminating containers for canning green billets prior to extrusion, and any such container may be used. A mild steel can is quite suitable.

After the billet is canned and thereby protected from exposure to an oxidizing atmosphere the chromium can safely be denitrided. This is accomplished by purging any oxygen gas from the container and contents at elevated temperature and lowering the partial pressure of nitrogen, as by evacuating the container or introducing therein a. flowing stream of hydrogen or inert gas such as argon, helium, neon, or krypton, while heating the canned billet to a temperature in the range of 900 to 1000 C., preferably 950 to 990 C. Above about 1000 C. there is danger of formation of Cr N; moreover, growth of the dispersed thoria or other refractory oxide is minimized by working below this temperature. The nitride gives up its nitrogen remarkably rapidly under the described conditions, and by continuing displacement of the atmosphere in the can substantially complete removal of the nitrogen can be accomplished in a few hours or even less.

After the billet has been denitrided it is hot-worked, by

. such conventional methods as extruding, While still enclosed in the container and in the presence of a non-reactive environment such as an argon or hydrogen atmosphere. The hot working should increase the density of th billet to substantially of theoretical. It is found that dense metal product so obtained is characterized by having a clean microstructure at 200 diameters magnification, and it can be further worked by such methods as rolling, forging and swaging to form products having excellent strength and ductility with a minimum of anisotropic properties.

In describing the products as having a clean microstructure the term clean is used in accordance with customary practice in metallographic examination of specimens. Ordinarily, the specimens upon which observations are made are in the form of mill products such as sheet. It has been found that sheet manufactured by processes involving the use of nitrided powders according to the present invention is particularly free from the presence of large quantities of non-metallic inclusions. The number of such inclusions which can be observed on a metallographically polished surface at 200x magnification is less than 100 particles per any one-inch square of area of a microphotograph taken at this magnifiication. In a preferred aspect, less than one percent of the observed particles will be greater than five microns in diameter.

By reason of their clean microstructures the solid metal products of this invention have excellent high-temperature strength properties which do not change upon working the metal in a particular direction, as for example by bending it, in the form of sheet, around small radii. The chromium nitride-containing powders are useful as intermediates in making the solid metal products and have the especial utility that they do not pick up oxygen upon exposure to air at temperatures below about 100 C.

The invention will be better understood by reference to the following illustrative example.

Example 1 A co-precipitated and calcined powder containing, by weight, about 76.1% NiO and about 22% Cr O and by volume, about 1.7% ThO was reduced under conditions to give a product in which the thoria average particle size was below about 80 millimicrons and carbides and metal oxides other than thoria were substantially absent. This powder was charged to an oxygen-free reactor and after the reactor and its contents were heated to about 750 C., pre-purified nitrogen was fed to the reactor at a rate of about 8 cubic feet per hour for 20 hours. This nitrogen was pretreated before being fed to the reactor by (1) bleeding in about 4% hydrogen by volume, (2) passing the nitrogen-hydrogen mixture over a catalyst to convert any oxygen present to water, and (3) removing said water in a drier. At the conclusion of the 20 hour treatment in which nitriding was effected, the reactor was cooled to room temperature under a slow nitrogen purge. The product, 2.7 lb. in weight, was recovered.

Analyses of the material were as follows:

Total O =.302% by weight ThO =2.35% by weight Specific surface=0.6 m. gm. N =3.87% by weight C=16 p.p.m.

S: 15 p.p.m.

Cr=20.01%, by weight Balance, substantially nickel.

X-ray analysis showed diffraction lines indicating the presence of the phases Ni, Ni-Cr alloy, ThO and CrN.

This powder was hydrostatically compacted into a 2-inch diameter billet under pressure of 60,000 pounds per square inch. The billet was placed in a close-fitting mild-steel can which was connected to a sintering furnace. The billet was evacuated and tested to assure that air would not leak into the billet. A steady flow of gettered hydrogen was introduced into the mild-steel can, and the billet was heated stepwise to 925 C. to initiate denitridation.

When the billet reached a temperature of 925 C., it was held at this temperature for 16 hours. At this temperature, denitriding 0f CrN proceeded rapidly and quantitatively. After 16 hours, the sintered billed was cooled to room temperature under hydrogen flow, and the gas inlet and outlet tubing sealed.

The billet was extruded at a temperature of 982 C. and a reduction ratio of 8:1, to a dense bar. The nitrogen content was 174 p.p.m. and the oxygen in excess of that present as ThO was nil. Upon rolling the extruded bar to a sheet of dispersion-hardened nickel-chromium alloy, a product having outstanding mechanical properties and free of impurities and microstructural defects was obtained. Thus, this product had a clean microstructure when examined metallographically at 200x magnification.

I claim:

1. A powder composition comprising as essential components an iron-group metal, chromium in the proportion of 10 to 40% by weight, based on the total weight of the composition, at least about 98% of said chromium being in the form of chromium nitride of the formula CrN, and about from 0.05 to 20% by volume, based on the composition without the nitrogen of the chromium nitride, of particles of a refractory metal oxide having a free energy of formation at 1000 C. greater than 103 kilocalories per gram atom of oxygen and an average particle size of less than about 80 millimicrons, said essential components being codispersed substantially uniformly in the powder particles, and said powder particles being substantially free of metal carbides and of metal oxides other than the refractory oxide.

2. A powder composition of claim 1 in which the irongroup metal is nickel and the refractory oxide is thoria.

3. In a process for producing a powder composition of claim 1, the steps comprising (1) providing a powder comprising as essential components an iron-group metal, chromium in the proportion of 10 to 40% by weight, based on the total weight of the composition, and about from 0.05 to 20% by volume of particles of a refractory metal oxide having a free energy of formation at 1000 C., greater than 103 kilocalories per gram atom of oxygen and said average particle size less than about millimicrons, said essential components being codispersed in the powder particles and said powder particles being substantially free of metal carbides and of metal oxides other than the refractory oxide, and (2) heating said powder in nitrogen at a temperature in the range from 650 to 890 C. until at least 98% by weight of the chromium present has combined with nitrogen to form chromium nitride of the formula CrN.

4. A process of claim 3 in which the iron-group metal is nickel and the refractory oxide is thoria.

5. In a process for protecting a dispersion-modified alloy against oxidation while processing by powder-metallurgy techniques, the steps comprising 1) providing a powder comprising as essential components an iron-group metal, chromium in the proportion of 10 to 40% by weight, and about from 0.05 to 20% by volume of particles of a refractory metal oxide having a free energy of formation at 1000 C. greater than 103 kilocalories per gram atom of oxygen and an average particle size less than about 80 millimicrons, said essential components being codispersed in the powder particles and said powder particles being substantially free of metal carbides and of metal oxides other than the refractor oxide, (2) heating said powder in nitrogen at a temperature in the range from 650 to 890 C. until at least 98% by weight of the chromium present has combined with nitrogen to form chromium nitride of the formula CrN, (3) compacting the product of step (2), at a temperature below about 50 C., to from 50 to 75% of theoretical density, (4) enclosing the compact in a non-contaminating container before it has come into contact at elevated temperatures with an oxygen-containing gas, (5) displacing the atmosphere in said container, at a temperature in the range of 900 to 1000 C., with a medium selected from the group consisting of vacuum, flowing inert gas, and flowing hydrogen, whereby nitrogen in the chromium nitride is removed, and (6) continuing said displacement until all said nitrogen has been removed.

6. A process of claim 5 in which, in step (1), the irongroup metal is nickel and the refractory oxide is thoria.

7. In a process for protecting a dispersion-modified alloy against oxidation while processing by powder-metallurgy techniques, the steps comprising 1) providing a powder comprising as essential components an iron-group metal, chromium in the proportion of 10 to 40% by weight, and about from 0.05 to 20% by volume of particles of a refractory metal oxide having a free energy of formation at 1000 C. greater than 103 kilocalories per gram atom of oxygen and an average particle size less than about 80 millimicrons, said essential components belng codispersed in the powder particles and said powder particles being substantially free of metal carbides and of metal oxides other than the refractory oxide, (2) heating said powder in nitrogen at a temperature in the range from 650 to 890 C. until at least 98% by weight of the chromium present has combined with nitrogen to form chromium nitride of the formula CrN, (3) compacting the product of step (2), at a temperature below about 50 C., to from 50 to 75% of theoretical density, (4) enclosing the compact in a non-contaminating container before it has come into contact at elevated temperatures with an oxygen-containing gas, (5) displacing the atmosphere in said container, at a temperature in the range of 900 to 1000 C., with a medium selected from the group consisting of vacuum, flowing inert gas, and flowing hydrogen, whereby nitrogen in the chromium nitride is removed, (6) continuing said displacement until all said nitrogen has been removed, and (7) the compact, while still enclosed in the non-contaminating container and in a nonreactive environment, is hot-worked to substantially theoretical desity.

8. A process of claim 7 in which, in the starting powder, the iron-group metal is nickel and the refractory oxide is thoria.

References Cited UNITED STATES PATENTS 2,972,529 2/ 1961 Alexander et a1.

5 FOREIGN PATENTS 598,331 2/1948 Great Britain.

CARL D. QUARFORTH, Primary Examiner 10 R. L. GRUDZIECKI, Assistant Examiner US. Cl. X.R. 

